Instant Teen - Just Add Nuts Manga by Haruka Fukushima

Natsumi Kawashima is a fifth grader who can't wait to grow up and get curves of her own. Well one day after eating a weird pack of pink nuts, which she took from a lady in street, her wish of being older came true when the next day she turn into a beautiful young woman. She later finds out that the nuts were a experiment by a college student named Dr. Yunosuku Morinomiya. And that eating the nuts causes her to turn into a young adult.